The laser is a device for the generation of coherent, nearly single-wavelength and single-frequency, highly directional electromagnetic radiation emitted somewhere in the range from submillimeter through ultraviolet and x-ray wavelengths. The word laser is an acronym for the most significant feature of laser action: light amplification by stimulated emission of radiation.
There are many different kinds of laser, but they all share a crucial element: each contains material capable of amplifying radiation. This material is called the gain medium, because radiation gains energy passing through it. The physical principle responsible for this amplification is called stimulated emission. It was widely recognized that the laser would represent a scientific and technological step of the greatest magnitude, even before T. H. Maiman constructed the first one in 1960. Laser construction generally requires three components for its operation: (1) an active gain medium with energy levels that can be selectively populated; (2) a pumping process to produce population inversion between some of these energy levels; and usually (3) a resonant electromagnetic cavity structure containing the active gain medium, which serves to store the emitted radiation and provide feedback to maintain the coherence of the electromagnetic field.
In a continuously operating laser, coherent radiation will build up in the cavity to a level set by the decrease in inversion required balancing the stimulated emission process with the cavity and medium losses. The system is then said to be lasing, and radiation is emitted in a direction defined by the cavity.
A common approach to converting the laser wavelength to half its value (for example, from 1064 nm to 532 nm), often used to convert infra-red lasers to lasers emitting in the visible part of the spectrum, is to use intra-cavity frequency up conversion (IC). The most common IC approach is to incorporate a second crystal, a non-linear optical crystal, correctly oriented for phase matching, inside the laser resonator, and to adjust the reflectivity of the cavity mirrors to maximize the wavelength converted laser light emission.
The lasers of the present invention use a new crystal material as the active gain medium. The new gain medium is trivalent neodymium-doped yttrium calcium oxyborate referred to herein as Nd.sup.3+ :YCa.sub.4 O(BO.sub.3).sub.3 or Nd:YCOB for easier reference. Patent Corporation Treaty (PCT) application numbered WO 96/26464 reports the growth of calcium gadolinium oxyborate, GdCOB, as the first element of a new family of borate crystals. The abstract for WO 96/26464 states, "The crystals are prepared by crystallising a congruent melting composition of general formula: M.sub.4 LnO(BO.sub.3 ).sub.3, wherein M is Ca or Ca partially substituted by Sr or Ba, and Ln is a lanthanide from the group which includes Y, Gd, La and Lu. Said crystals are useful as frequency doublers and mixers, as an optical parametric oscillator or, when partially substituted by Nd.sup.3+, as a frequency doubling laser." Although, the general formula might be interpreted to include various Nd-doped crystals, the PCT application, WO 96/26464, only demonstrates and claims Nd-doped GdCOB or LaCOB. Additionally, the subject inventors have discovered that the orientation of axes and angles for the demonstrated crystals disclosed in WO 96/26464 are not efficient for a self-frequency doubling laser. More importantly, WO 96/26464 does not demonstrate nor claim any method nor apparatus for using Nd-doped YCOB as a self-frequency doubling laser.
In the prior art, there are no disclosures of Nd:YCOB as an active gain medium or as the gain medium in a harmonic generation laser. As a member of the oxyborate family of crystals, the non-hygroscopic YCOB crystal possesses nonlinear optical properties and when doped with Nd.sup.3+ ions, the new crystals have the advantage of combining the active gain medium and the nonlinear frequency conversion medium in a single element. Self-frequency doubled (SFD) lasers are an attractive alternative to conventional intra-cavity frequency doubling with a separate nonlinear crystal such as potassium titanyl phosphate (KTP), as disclosed in U.S. Pat. No. 4,942,582. A SFD laser incorporates lower reflection, absorption and scattering losses and leads to a simpler and more robust resonator design. With the addition of diode-pumping, the Nd:YCOB laser provides a new type of compact, high-powered, visible laser light source.
Trivalent neodymium-doped crystalline laser systems producing optical radiation are reported. U.S. Pat. No. 4,942,582 supra, discloses a single frequency solid state laser having an active gain medium which comprises a block of neodymium doped crystals of vanadium oxide (YVO.sub.4), garnet (YSGG) and borate (YAB) in combination with a separate frequency doubling crystal of KTP (potassium titanyl phosphate, or KTiOPO.sub.4); this invention overlooked the self-frequency doubling possibilities of the Nd:YAB crystal. U.S. Pat. No. 5,058,118 disclosed that a single crystal of neodymium doped borate (Nd:YAB) was useful as a self-frequency doubling minilaser generating a 0.532 .mu.m (green light) and 0.660 .mu.m (red light) laser beam. However, this laser configuration suffers from poor optical quality and self-absorption at 530 nm as disclosed in J. Appl. Phys., Vol. 66, pp. 6052-6058, 1989.
More recently, the approach to generating high power, visible laser light has been to use nonlinear optical crystals to convert near-infrared radiation to the visible portion of the spectrum via second harmonic generation (SHG) (sometimes termed frequency doubling and used interchangeably, herein). This process generates a harmonic wavelength that doubles the frequency of the fundamental wavelength. Since the SHG conversion efficiency is a function of the fundamental laser beam intensity, the nonlinear crystal is often placed inside the cavity of a low-power continuous wave laser to benefit from the higher intracavity fundamental beam intensity. This technique is discussed in U.S. Pat. No. 5,610,934 and U.S. Pat. No. 5,751,751 which provides an example of frequency doubling when neodymium doped crystals of vanadium oxide (YVO.sub.4) or (GdVO.sub.4) are bonded to non-linear crystals of potassium niobate (KNbO.sub.3) or .beta. barium borate (BBO). A fundamental beam of about 914 nm is frequency doubled to produce blue laser light at about 457 nanometers (nm) or (0.457 .mu.m).
U.S. Pat. No. 5,802,086 discloses a continuous wave (cw) microlaser with a highly absorbing solid-state gain material, preferably neodymium-doped yttrium orthovanadate (Nd:YVO.sub.4) that lases at two fundamental wavelengths and are frequency-mixed in a nonlinear crystal oriented within the cavity to generate a third wavelength which maybe difficult to generate directly or by frequency doubling.
Popular host crystals including garnet, especially yttrium aluminum garnet (YAG) and yttrium orthovanadate (YVO.sub.4) are discussed in the prior art. However, the search for smaller, less expensive, more powerful, multifunctional lasers continues. The discovery of a new class of laser hosts, the oxyborates, makes possible the combination of linear and nonlinear optical properties in a single active medium. More particularly, the neodymium-doped oxyborate crystal (Nd:YCOB) of the present invention generates a near infrared fundamental light which can be self-frequency doubled to provide a compact, efficient, source of visible green laser light.